This invention relates generally to variable-pitch rotors and more particularly to control mechanisms for such rotors.
Aircraft powerplants are typically used to drive thrust-generating airfoil elements such as propellers or fan blades. It is known to vary the angle of incidence (i.e. “pitch angle”) of the airfoil elements relative to the rotating hub carrying them, in order to provide the maximum possible propulsive efficiency at various flight conditions.
A common method of pitch control employs a hydraulic actuator which changes the blade pitch angle in response to pressurized fluid flow. The actuator may move the blade through pitch angles from “coarse” to “fine” and may also provide pitch angles suitable for ground operation.
For safety reasons, it is important to limit the blade pitch angle during flight. This avoids overspeeding the powerplant, or imposing excessive structural loads or unexpected yawing moments to the aircraft. A typical prior art variable-pitch rotor includes a mechanical pitch lock which limits the blade pitch angle in the case of actuator failure. Pitch locks can be complicated and themselves subject to failure.
It is also known to provide variable-pitch rotors with counterweights. The counterweights provide a countervailing force that drives the blades to a safe pitch angle in case of actuator failure. However, these are typically mounted to the individual blades and therefore limit design flexibility.
Accordingly, there remains a need for a pitch control mechanism incorporating counterweights not directly mounted to the blades.